Pneumonia/ ID

Community Acquired Pneumonia

Community-acquired pneumonia refers to pneumonia that begins when the patient is outside the hospital, or within the first 48 hours of hospital admission. Patients previously categorized under "healthcare-associated pneumonia" are now considered to have community-acquired pneumonia due to evidence that the previous categorization fails to accurately identify patients harboring drug-resistant organisms.

Differential Dx

Differential DiagnosisCluesDiagnostic Tests
Pulmonary EmbolismWedge-shaped pulmonary infarcts may masquerade as pneumonia, unimpressive pulmonary infiltrates, right ventricular strain pattern on EKG/Echo, signs/symptoms of DVT (leg swelling, pain)CT angiography
PJP (Pneumocystis jiroveci pneumonia)HIV (if the diagnosis is known), chronic steroid use (>15 mg prednisone for >3 weeks), chemotherapy/immunosuppressive drugs, diffuse interstitial infiltratesHIV serology, bronchoscopy with BAL sent for fungal stain & PCR
Endemic Fungal Pneumonia or CryptococcusOften more indolent than bacterial pneumonia, radiologic pattern often nodular, exposure to endemic locations, bird/bat droppings, soil exposureUrine antigens (e.g., blastomycosis, cryptococcus), CT scan, bronchoscopy
Invasive AspergillosisNeutropenia (especially >10 days), high-dose steroid (e.g., pulse therapy for vasculitis)CT scan, Beta-D-Glucan, galactomannan, bronchoscopy
Septic Pulmonary EmboliDiffuse infiltrates which tend to cavitate, often bacteremic with Staphylococcus aureus, often seen in patients with intravenous drug useCT scan, echocardiogram
DAH (Diffuse Alveolar Hemorrhage)Hemoptysis (only 50% of patients), diffuse infiltrates, renal failure or active urinary sediment (hematuria), falling hemoglobin, may have previously diagnosed rheumatologic diseaseUrinalysis, markedly elevated ESR & CRP, bronchoscopy, serologies (e.g., ANCA)
AEP (Acute Eosinophilic Pneumonia)Blood eosinophils over ~300/uL (unusual for severe pneumonia), younger adults with severe pneumonia often requiring intubation, sometimes inhalational exposure (esp. recent-onset smoking)Bronchoscopy
COP (Cryptogenic Organizing Pneumonia)Often more gradual onset than usual pneumonia, refractory to antibioticsTissue biopsy
Drug- or Radiation-Induced PneumonitisExposure to drug implicated in causing pneumonitis (e.g., amiodarone, chemotherapeutics)Diagnosis of exclusion
Flare of Interstitial Lung DiseaseHistory of chronic pulmonary limitation, prior diagnosis of interstitial lung disease (often idiopathic pulmonary fibrosis)CT scan, diagnosis of exclusion
Exacerbation of COPDCOPD exacerbation may clinically mimic pneumonia including hypoxemia, fever, and sputum production; isolated COPD exacerbation should lack infiltrates on chest imagingIsolated COPD exacerbation: lack of infiltrates on chest imaging, relatively low procalcitonin level
Atelectasis (+/- non-pulmonary infection)Chest X-ray isn't impressive, clinical illness severity is disproportionate to the X-ray abnormalities, lack of prominent pulmonary symptomsCT chest
Aspiration PneumonitisHistory of aspiration or swallowing problems, repeated episodes of pneumonia with rapid recovery, infiltrates located in dependent lung segmentsCXR, rapid clearance of CXR, procalciton

Diagnosing CAP

The diagnosis of CAP is generally based on three lines of evidence:

  • imaging evidence of a chest infiltrate (like CXR, CT, ultrasound)
  • systemic inflammation (symptoms: night sweats, rigors, fevers; signs: fever or hypothermia; labs: leukocytosis, left-shift, elevated C-reactive protein, elevated procalcitonin)
  • localizing signs and symptoms (symptoms: dyspnea, cough, sputum production, pleuritic chest pain; signs: hypoxemia, tachypnea, abnormal lung ascultation).

There can be atypical presentations of CAP, particularly in elderly patients who may present with non-pulmonary complaints such as falling, delirium, or sepsis. When in doubt, it is recommended to get cultures and start antibiotics for pneumonia

CT scan can assist in the diagnosis of pneumonia, especially in patients with chronic lung disease and those who are chronically abnormal on chest X-ray. CT scan is more sensitive than chest X-ray and can detect pneumonia that may be missed in certain situations

Laboratory Testing

Following a diagnosis of pneumonia, various lab tests can be done such as:

  • blood cultures (recommended for severe pneumonia)
  • sputum for gram stain & culture for bacteria
  • sputum for fungus culture & smear
  • urine legionella antigen
  • urine pneumococcal antigen
  • nares PCR for MRSA
  • nasopharyngeal PCR for COVID and other viruses
  • procalcitonin and C-reactive protein (CRP)
  • HIV screening test


Bronchoscopy is rarely useful in the management of community-acquired pneumonia, but occasionally useful to exclude a non-infectious pneumonia mimic (like diffuse alveolar hemorrhage, eosinophilic pneumonia), or to exclude an unusual infection, primarily fungal pneumonia or pneumocystis jirovecii pneumonia.

Who Needs ICU?
The IDSA suggest ICU admission for severe pneumonia patients

  • Major criteria: respiratory distress requiring mechanical ventilation and septic shock, with
  • Minor criteria: respiratory rate >29 breaths/min, hypotension requiring volume resuscitation, PaO2/FiO2 <250, and other signs of physiological stress or impaired immune response

Common errors in pneumonia triage include basing decisions solely on the amount of oxygen the patient requires or relying on CURB65 and PORT scores, which are not specifically designed for ICU admission decisions.

CURB-65 Pneumonia Severity Score

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Antibiotic Selection for CAP

Antibiotic SelectionTips
Atypical CoverageInclude atypical coverage in empiric antibiotic regimen for severe pneumonia
Legionella causes ~10-15% of severe pneumonia, not covered by broad beta-lactams
Azithromycin: Solid track record in pneumonia
- Well-tolerated and safe, no concern for QT interval prolongation
Doxycycline: Covers organisms acquired from animal contact
- Potential activity against MRSA
Beta-Lactam BackboneCeftriaxone: Excellent choice for most patients
- Controversy over dose (1 or 2 grams IV daily) and obese patients
- Safe to use in most patients with penicillin allergy
Antipseudomonal beta-lactam (piperacillin-tazobactam or cefepime):
- Generally safe for patients with penicillin allergy
- Double coverage generally not necessary for pseudomonas, unless highly resistant to beta-lactams
MRSA CoverageUncommon cause of community-acquired pneumonia, most patients do not need MRSA coverage
- Tailored strategy for MRSA coverage based on risk factors
Linezolid: Arguably first-line therapy for MRSA pneumonia, superior lung penetration, no nephrotoxicity
Vancomycin: Traditional option, increasing resistance over time, consider alternative if borderline sensitivity or MIC >2 mcg/mL
Ceftaroline: Active against MRSA, may be superior to vancomycin, limited evidence available
Anaerobic CoverageNot needed for pneumonia, unless empyema or lung abscess is present

Common Antibiogram

Large volume fluid resuscitation should generally be avoided as it can worsen hypoxemic respiratory failure. Vasopressors can be used to stabilize patients if needed. Fluid should only be used if there is organ hypoperfusion or refractory hypotension, and the patient's history and evaluation indicate true volume depletion.

  • Respiratory Support
    High-Flow Nasal Cannula (HFNC) may be beneficial for patients with significant work of breathing and/or tachypnea, as it can reduce work of breathing, provide oxygenation support, and promote secretion clearance. BiPAP is typically avoided due to its potential to cause retention of secretions. Endotracheal intubation is usually considered after trying HFNC, particularly for refractory hypoxemia or progressively worsening work of breathing.
  • Steroids
    Steroids may reduce the length of stay and risk of intubation in critically ill patients with Community-Acquired Pneumonia (CAP), although their impact on mortality is unclear. Contraindications include suspicion of fungal or tuberculosis pneumonia, immunocompromise, history of steroid-induced psychosis, and other specific cases.
  • Pleural Effusion
    Pleural effusion and empyema are common in severe pneumonia. These should be evaluated using bedside ultrasonography. Management is driven by ultrasonographic features, which determine whether to follow with daily ultrasonography, drain with thoracentesis, or place a pigtail catheter.
  • Treatment Failure
    Treatment failure may be indicated by lack of clinical improvement within ~3 days, persistent or rising procalcitonin or CRP over several days, or ongoing deterioration in oxygenation and infiltrates >24 hours after antibiotics.

Duration of Treatment
A time-based strategy suggests 5-7 days of treatment, with longer periods for certain conditions. A procalcitonin-based strategy suggests discontinuing antibiotics when procalcitonin levels fall below certain thresholds. Both strategies can be considered when determining the duration of treatment.

Antibiotic table for various infections

Clinical SyndromeMost likely pathogensReasonable initial empiric antibiotic agents
Community-acquired pneumonia inpatient therapyPneumococcus, H flu, M Catarrhalis, atypicalsCeftriaxone plus azithromycin; respiratory quinolone (levofloxacin/moxifloxacin). If risk factors for resistant organisms (cefepime, piperacillin-tazobactam, meropenem, or imipenem-cilastin). Add vancomycin or linezolid if risk for MRSA. With severe COPD, consider using an antibiotic with pseudomonas coverage instead of ceftriaxone.
Hospital-acquired pneumonia (non-ventilator acquired)MRSA, MSSA, pneumococcus, gram negatives including pseudomonasEither piperacillin-tazobactam, cefepime, ceftazidime, levofloxacin, ciprofloxacin, imipenem, meropenem. Add vancomycin or linezolid if IV antibiotics within 90 days, prevalence >20% or not known, high risk of mortality, or previous detection of MRSA.
Aspiration pneumonia in an alcoholicAnaerobes, gram negatives, pneumococcusAmpicillin/sulbactam; piperacillin/tazobactam; clindamycin plus ceftriaxone; metronidazole plus ceftriaxone. New guidelines only need anaerobic coverage if empyema or abscess.
IVDA with fever and pleuritic chest painStaph aureus including MRSA, enterococcus, gram negativesVancomycin
Nursing home patient with UTI + FoleyGram negatives including pseudomonas, rarely enterococcusCeftazidime, cefepime, aminoglycosides, ciprofloxacin. Vancomycin if urine gram stain gram +s.
Diabetic foot ulcer (malodorous)Anaerobes, gram negatives, gram positives including possible MRSAPiperacillin/tazobactam plus vancomycin for MRSA
Intravenous catheterMRSA, MSSA, occasionally gram negatives including pseudomonasVancomycin but add pseudomonas coverage if acutely ill
Cellulitis requiring hospitalizationGram positives including streptococcus and staph aureus including MRSAIf purulent cellulitis, need to cover MRSA. If non-purulent, then likely streptococcus. If purulent, usually vancomycin for MRSA, but if non-purulent, cefazolin for strep and MSSA.